Silver-iron batteries are well known in the art, and are taught by Brown, in U.S. Pat. No. 4,078,125, and Buzzelli, in U.S. Pat. No. 4,383,015. These patents teach the use of perforated silver sheet or expanded silver screen supports containing active silver material for positive plates, either sulfurized iron oxide negative plates according to the teachings of Jackovitz et al., U.S. Pat. No. 4,356,101, or sintered metallic iron negative plates. Brown and Buzzelli, in the above patents, both taught a multi-ply separator between positive and negative plates. The separator contained alternating porous and microporous sheets of polypropylene. One of the sheets, made of 60% to 90% porous, non-woven polypropylene, having 4 micron to 30 micron pores, was placed next to the silver electrode. The microporous polypropylene had pores of from about 0.05 micron to 3 micron diameter. Total separator thickness was generally about 0.050 to 0.075 cm (0.020 to 0.030 inch).
The silver-iron battery is now generally considered more stable than the silver-zinc battery. The silver-zinc battery has always presented major problems of internal electrical shorts due to zinc dendritic growth from the negative plate through the separator system. The soluble silver in both silver-zinc and silver-iron systems has also caused some problems. One problem has been the tendency to form a silver conducting film on the separators, which could allow shorting. Both battery systems are quite expensive, and are usually restricted to applications where the energy density of the battery is critical to the total system mission. An example of such an application is the propulsion system power source for underseas vehicles.
A number of patents have issued on improved battery separator materials for use in silver batteries, most for use with silver-zinc couples. Langer et al., in U.S. Pat. Nos. 3,749,604 and 3,953,241, taught a porous, caustic resistant, polymeric support, such as polypropylene, polytetrafluoroethylene or the like, coated on at least one side with a polymeric matrix, such as polysulfone having pore diameters of from about 5 microns to 50 microns, containing inorganic filler particles. This separator was found useful for silver-zinc or silver-iron couples. Moshtev et al., in U.S. Pat. No. 4,234,623, taught a five layer battery separator for alkali accumulator batteries. The separator contained, in order: inert, outside polyester layer; cellulose material, such as cotton, impregnated with methacrylic acid; central, irradiated, activated, low density (0.925 g./cm.sup.3) polyethylene film, about 35 microns thick, graft polymerized with methacrylic acid, where there was a high degree of grafting, i.e., 80.sup.+ %; cellulose material, such as cotton, impregnated with methacrylic acid; and inert, outside polyester layer. Minimum separator thickness, prior to any pressing, was about 495 microns (0.02 inch).
Nagamine et al., Japanese Pat. Kokai No. 54-50829 [Application No. 52-116415], relates to separators for silver-zinc mercurate button cells. The separator contained, in order outer cellophane film; porous, synthesized, high-molecular weight polyethylene, polypropylene, polytetrafluoroethylene, or polyester film; and outer cellophane film. Another embodiment of the separator contained one piece of cellophane film, and either one or two pieces of porous, synthesized, high-molecular weight polyethylene, polypropylene, or polytetrafluoroethylene film. Nagamine et al., Japanese Pat. Kokai No. 57-95069 [Application No. 55-171763], relates to a laminated separator for silver-zinc button cells. The laminated separator contained, in order: irradiated, polyethylene film, graft polymerized with acrylic acid or meth-acrylic acid (where there was a 20% to 40% graft rate), next to the silver anode material; cellophane film, 20 micron to 30 micron thick; irradiated polyethylene film, graft polymerized with acrylic acid or meta-acrylic acid (where there was a 45% to 90% graft rate); and outside single or double cellophane sheets, each 20 microns to 30 microns thick. The prior art was characterized in Table 1 as cellophane film sandwiched by two pieces of graft polymerized polyethylene films with equal graft rates. Adams et al., in U.S. Pat. No. 4,144,301, taught a deposited film or shaped envelope separator, for use in electrolytic cells. The separator contained a single sheet of irradiated, low density polyethylene, or polypropylene, graft polymerized with acrylic acid or meth-acrylic acid.
While many of these separator materials may be somewhat resistant to oxidation by divalent silver ions, many of them may also allow cellophane degradation by silver ions, and most would allow long term diffusion of silver ions into the electrolyte. Cellulose has been found to be subject to hydrolytic attack and to degrade in electrolyte at temperatures over 45.degree. C. Polypropylenecellophane combinations have been found to allow large scale silver mirror build-up, which over a long period of time could cause shorts between any silver-metal battery couple. The separator can be the weakest component in a sophisticated battery, and generally is the primary life limiting source for silver-iron batteries, since the iron electrode is stable (unlike zinc electrodes).
A superior separator system for a long-life, high energy density, silver-iron battery, utilizing high discharge rate silver electrodes, requires a separator that allows rapid passage of electrolyte ions; helps protect any cellulose material present from silver ions; provides stable performance in concentrated alkaline solution over a temperature range of from about 0.degree. C. to 45.degree. C., and preferably up to 70.degree. C.; eliminates oxidation by divalent silver ions or oxygen; absorbs electrolyte; is ultra thin, i.e., less than about 0.05 cm (0.02 inch); and most importantly, prevents long term silver ion migration. What is needed is a sophisticated separator system capable of achieving all these goals.
It is an object of this invention to provide a separator system for long life, high energy density, high discharge rate silver-iron batteries, where the separator is thin, will resist heat and chemical degradation, and will markedly decrease the possibility of electrical shorting.